Science Musings Blog

Monday, October 25, 2010

A hint of gooseberries

That was one of the qualities of the wine I had for dinner last evening, "a hint of gooseberries." Not, mind you, that I detected the labeled hint with my dysfunctional nose. Nor was the wine one I had purchased myself. Having no sense of smell -- and therefore little sense of taste -- I tend to buy cheap plonk that wouldn't know a gooseberry if it saw one.

I'm OK with sensations that reside on the tongue: sweet, sour, salty, bitter. Our taste buds -- tiny onion-shaped receptors embedded in the tongue -- are multipurpose chemical processors capable of sorting out an assortment of molecular stimuli. For example, the salty pleasure I derive from anchovies begins with sodium chloride molecules, NaCl, approaching a taste cell. The atoms in the molecules disassociate, and sodium ions enter the cell through special channels on its surface. The accumulation of sodium ions in the cell enables calcium ions to enter, too. This prompts the release of neurotransmitters that trigger adjacent nerve cells. A message zips to the brain. Yum, I'll have more anchovy pizza, please. And, by the way, I think the wine has gone off.

The sense of smell is rather different. The 100 million olfactory receptors in the nose are bare nerve endings; no fancy buds to do complex biochemistry. As the chemist P. W. Atkins said: "In essence, the brain is exposed in the nose." Smell is raw and primitive, a link to our premammalian past.

The triggering mechanism is lock-and-key. Molecules of a certain shape fit nooks and crannies on nerve-cell proteins, causing the cell to send its own unique message up the line -- musk, citrus, oak, attar of roses. It's all geometrical.

For almost everyone but me.

Presumably molecules of certain shapes find their nests in my nose, and presumably the appropriate signals go whizzing up to the frontal lobes of my brain. But whoever operates the equipment there that registers conscious sensation has been asleep at the switch for as long as I have memory.